1. Field of the Invention
The present invention relates to a plastic encapsulated semiconductor device which is suitable for a semiconductor device having a current capacity of about 5 to 50 amperes.
2. Description of the Prior Art
A can type diode has been usually used as a semiconductor having a current capacity of about 5 to 50 amperes as shown in FIG. 1(a).
This has the characteristics that the electric characteristic of the semiconductor device is not deteriorated for a long time because the semiconductor device is sealed in a can. However, it has disadvantages that the parts are expensive and the method of preparing it is complicated resulting in low productivity in mass production and the size and the weight are large.
Heretofore, the connected lead or the connected metal plate so called lead frame have been used for the mass production of I.C., transistors, thyristors or diodes which have a small capacity of less than 5 amperes.
FIG. 1(b) is a plan view of the typical lead frame (100) which has been widely used for preparing transistors or thyristors devices.
The conventional lead frame (100) usually has a thickness of about 0.2 mm to 1 mm and has been prepared by punching a metal plate made of copper or a copper alloy by a press with a metal mold.
However, the lead frame used for semiconductor devices having the current capacity of more than 10 amperes should have the lead frame having more than 2 mm of a thickness of about ten times to that of the conventional lead frame (100) in order to radiate heat generated in a semiconductor element having a pn-junction with high efficiency and to reduce the resistance to the current passed through the lead frame and to give suitable mechanical strength depending upon the size of the semiconductor elements.
When a lead frame is prepared from a metal plate with a metal mold, the side surface contacting the cutting edge of the metal mold is substantially perpendicular to the plane of the lead frame to clearly leave a ridge and the dull of curved ridge is not found, in the case that the lead frame is thin.
However, the dull of curved ridge appears in the case that the lead frame is thick, to form a large curved surface at the ridge between the plane surface and the side surface of the lead frame.
When the semiconductor devices are prepared by using the lead frame having large curved surface at the ridge, various disadvantages are caused.
Referring to FIGS. 2 to 9, the semiconductor device having the conventional thick lead frame having the dull of curved ridge for the plastic encapsulated diode device having relatively large current capacity will be illustrated.
In the description, the lead frame used in the present invention is referred to as a connected metal plate so as to discriminate it from the conventional lead frame.
A thick metal plate (2.5 mm) for mounting a diode element (for 15 amperes) is prepared by punching a master metal plate made of copper or copper alloy with a metal mold.
FIG. 2(a) is a schematic view of a connected metal plate (200) and FIG. 2(b) is a sectional plan view taken along the line II.sub.b -II.sub.b of FIG. 2(a).
In FIGS. 2(a), (b), the metal plates (base) (1) are formed on the connected metal plate (200) and are connected in series in one direction with connecting arms (3) having the side surface of the metal plate (1).
In the connected metal plate (200), the large dull (5) is formed in the punching direction because the metal plate is punched by the cutting edge of the metal mold in the arrow line direction (4) in FIG. 2(b).
The condition of the dull (5) at the sectional surface (6) is also shown in the schematic view of FIG. 2(a).
FIG. 3 is a schematic view showing the condition that the diode element (7) is soldered on the metal plate (1) of the connected metal plate (200) having the dull (5) with a solder, and a lead (8) is soldered on the diode element (7) with a solder.
FIG. 4 is a sectional plan view taken along the line IV--IV of FIG. 3.
In FIG. 4, the references (71), (72) respectively designate solder layers.
The diode element (7), the part of the metal plate (1) of the connected metal plate (200) and the lead (8) are encapsulated with plastic (9) to form a plurality of the plastic encapsulated diodes on the connected metal plate (200) as shown in the schematic view of FIG. 5.
FIG. 6 is a sectional front view taken along the lines VI-VI of FIG. 5.
The plurality of the plastic encapsulated diodes on the connected metal plate (200) are cut at the connecting arms (3) to obtain the separated plastic encapsulated diodes.
Referring to the sectional view of FIG. 7, the method of cutting the plastic encapsulated diode devices on the connected metal plate (200) will be illustrated.
As shown in FIG. 7, the connected metal plate (200) is put on a base metal mold (10) so that the surface having the dulls (5) on which the plastic encapsulated part (9) of the connected metal plate (200) is not formed, is contacted with the base metal mold (10).
The push metal mold (12) for deciding the positions of the upper cutting edges (11) is put on the connected metal plate (200) near the plastic encapsulated parts and the upper cutting edges (11) are pushed in the arrow line direction (13) to cut the connecting arms (3) of the connected metal plate (200).
In this condition, the upper cutting edges (11) are adjusted so as to fit between the base metal mold (10) and the base metal mold (10).
The cutting method is usually carried out with high efficiency.
FIG. 8 shows the condition of the change of the connecting arm (3) cut by the cutting method. That is, FIG. 8 is a front view of the plastic encapsulated diode device formed on the connected metal plate (200) from the cut surface of the connecting arm (3).
Before the cut, the connecting arm (3) have the dulls (5) formed by punching a metal plate by a metal mold to prepare the connected metal plate (200).
The dulls (14) are formed on the connecting arms (3) by the punching force of the upper cutting edges (11) to the connected arms (3) in the arrow line direction under putting the connected metal plate (200) on the plane surface of the base metal mold (10), whereby the voids (15) are formed in the plastic encapsulated part. The formation of the voids (15) causes the deformation of the metal plate (1) whereby the strain is given to the diode element (7) soldered on the metal plate (1) and sometimes the diode element (7) is broken to be deteriorated.
The moisture in the atmosphere is permeated through the void (15) at the boundary surface between the plastic sealed part (9) and the metal plate (1) to cause the disadvantages of the deterioration of the electric characteristics of the diode element (7).
When the semiconductor devices are prepared by using the conventional connected metal plate (200), the above-mentioned serious disadvantages are caused in the semiconductor devices.